A wide variety of different fibers are used to produce fiber products, for example, cotton fiber, cellulose fiber, synthetic fiber, and combinations of these fibers.
In general, a loosely continuous layer of fibers is put on the forming wire with a card or a fiber distributor in a thickness of approximately, for example, 25 mm. From the forming wire, the fiber layer continues to a roller, where it is rolled into a thinner layer with a thickness of approximately, for example, 0.1 mm. This rolled down fiber layer then passes through one, or several rollers which, depending on type and application, warm and crosslink the fibers, roll on a pattern, and cool down the fiber layer.
There is an open section between the outlet end of the forming wire and the roller where the now freely hanging fiber layer is not supported. The layer must therefore be tightened in order for it to be sufficiently stabilized for being fed into the roller without break-downs or production faults occurring. This tightening means that the fiber layer is stretched and becomes thinner. However, the layer has a natural tendency to be stretched most in those areas where it is already thinnest. The structure of the finished product therefore varies in density and strength, and the surface appears uneven and blotchy. This means that the product cannot satisfactorily meet today's high quality requirements as with regard to the quality and appearance of such products.
Even if the fiber layer is stretched while it passes this open section, the freely hanging layer continues to be very unstable, and this lack of stability limits the production speed of conventional systems. There are thus no known systems of this type which can operate at production speeds of over 200 m/min.
The fiber layer has a large content of air which, during the compression process in the roller flows out via, amongst others, the freely hanging fiber layer. When the production speed exceeds a certain limit, the air flow has such a force that the relatively loosely connected fibers in the freely hanging fiber layer are not able to resist the air pressure sufficiently. In this case, the fiber layer could more or less be blown apart by the air, thus making further rolling difficult and reducing the quality of the finished product.
In order to rectify this drawback, attempts have been made to precompress the fiber layer already during transport on the wire by using a second wire, inclined towards the transport direction, and placed above the fiber layer of the first wire. The fiber layer is, in this way, successively pressed together between the two wires during transport to the outlet end of the first wire.
Such plants are known from U.S. Pat. No. 4,146,564 and U.S. Pat. No. 4,476,175. However, it has become apparent that after the purely mechanical compression in these plants, the fiber layer has a tendency to spring back into shape to such an extent when passing this open section, that the following heat treatment process for binding the fibers together has not been able to run satisfactorily.